1. Technical Field:
The present invention generally relates to data telemetry systems for downhole sensors and other equipment and in particular to systems employed for data telemetry over power cables for an electrical submersible pump. Still more particularly, the present invention relates to employing phase-to-phase signaling over three phase power cables to an electrical submersible pump for downhole telemetry.
2. Description of the Related Art
Various downhole telemetry systems for returning electrical signals from sensors within a borehole have been proposed and/or utilized. Several such systems employ the three phase power cable transmitting three phase power downhole to an electrical submersible pump or other load device for transmitting the telemetry signals. Within this type of system, generally the signaling arrangement treats all three phases of the power conductor cables as a single conductor and requires a ground reference for the return path, typically the production tubing and well casing.
Systems which treat the three phase power cable as a single conductor, such as U.S. Pat. Nos. 5,515,038 to Smith and 5,539,375 to Atherton, generally employ direct current (DC) offsets to the alternating current (AC) power transmitted by the cable. Additionally, many systems which treat the three phase power cable as a single conductor cannot tolerate a ground referenced electrical power system, either intentional or inadvertent. Thus, for example, if one phase or conductor of the power system should accidentally be shorted to ground, the aspects of downhole components which rely on the three-phase power (e.g., the pumping capability of the ESP) continue to operate while the telemetry system is disabled.
It would be desirable, therefore, to provide a telemetry system employing three-phase power conductors for an ESP for the data signals without requiring a return or ground reference conductor, but fault-tolerant with respect to unintentional grounding of one phase.
It is therefore one object of the present invention to provide an improved data telemetry system for downhole sensors.
It is another object of the present invention to provide improved data telemetry over power cables for an electrical submersible pump.
It is yet another object of the present invention to provide a mechanism for employing differential signalling over three phase power cables to an electrical submersible pump for downhole telemetry.
The foregoing objects are achieved as is now described. A three phase power cable is employed to transmit data encoded in a phase-to-phase manner within a high frequency carrier signal transmitted simultaneously with power transmitted from surface equipment to downhole components.
The high frequency carrier signal is transmitted on at least two conductors of the power cable, with the signal carried on at least two phases or conductors modulated between at least two conductors to encode data between the two phases. Any modulation technique may be employed. Multiple phase-to-phase signals may be encoded and transmitted between different pairs of the power cable conductors, or the same data may be encoded and transmitted in multiple phase-to-phase signals in either a single-ended or differential manner. The high frequency carrier may be employed to transmit data from sensors downhole to the surface equipment, control signals from the surface equipment to the downhole components, or both.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.